Water dispersion-type acrylic pressure-sensitive adhesives using water dispersion-type acrylic polymers do not use organic solvents unlike solvent-type acrylic pressure-sensitive adhesives. Therefore, the former pressure-sensitive adhesives have advantages that they are desirable in view of environmental health and are also excellent in view of solvent resistance. In general, in the pressure-sensitive adhesive layers using acrylic pressure-sensitive adhesives, acrylic polymers have low adhesiveness and terminal peeling resistance (a property that a terminal part is hardly peeled off after attached to a substrate to be adhered) and hardly exhibit the properties, so that the adhesiveness and terminal peeling resistance thereof are enhanced by means of emulsions containing tackifying resins. However, emulsions containing tackifying resins sometimes use organic solvents at their production steps for the purpose of lowering solution viscosity and melt viscosity of the tackifying resins [cf. JP-A-2002-285137 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)]. In this regard, in order to produce emulsions containing tackifying resins without using any organic solvent, emulsification should be carried out under high temperature and high pressure. In the case of a resin having a high softening temperature (e.g., a resin having a softening point of 130° C. or higher), the method is very difficult to apply and the kind and amount of an emulsifier to be used are largely limited in many cases. On the other hand, in the case of a resin having a low softening temperature (e.g., a resin having a softening point of less than 130° C.), there is a case that the resin can be emulsified by melting it through heating with using no solvent but in the case of using only the tackifying resin having the low softening temperature, it is considered that a high adhesiveness and excellent terminal peeling resistance and thermal resistance are hardly obtained and the applications of the pressure-sensitive adhesive sheets prepared are limited.
As mentioned above, in order to enhance workability at the time of lowering the solution viscosity and melt viscosity of the tackifying resin having a high softening temperature and satisfy pressure-sensitive adhering properties by broadening flexibility of the kind and amount of the emulsifier to be used, there is a case that the tackifying resin is dissolved or melted using an organic solvent. Usually, the solvent is removed by evaporation under reduced pressure but there is a problem that the above organic solvent remains in a final product although the amount thereof is minute. As such an organic solvent, in view of workability and cost, an aromatic hydrocarbon-based organic solvent such as toluene is generally used. However, since aromatic hydrocarbon-based organic solvents are highly toxic, there is a fear of their influence on environment when pressure-sensitive adhesives and pressure-sensitive adhesive sheets with remaining aromatic hydrocarbon-based organic solvents are used. In particular, there is also a fear of the influence thereof on human bodies in the applications in closed spaces, such as interior materials for automobiles and house building materials.